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<html> <head> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../../css/ccna.css"> <script src="../../../../../js/framesetHelper.js" language="JavaScript"></script><script language="JavaScript"> var pageLoaded = false; var current = ''; var currentChapter = 'knet-v214aCH47510'; function launchReference(rioID, rloID, chapterID) { contentID = "knet-v214aCV47051"; dash = contentID.indexOf('-'); sub1 = contentID.substring(0,dash); sub2 = contentID.substring(dash+1); numbackParams = "CHAPID=" + chapterID + "/RLOID=" + rloID + "/RIOID=" + rioID; ContentRef = "../../../../../"+ numbackParams + "/"+ sub1 +"/"+ sub2 +"/"; eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_pf.html"'); } function PrevPage() { top.currentSupport--; if (top.currentSupport <= 0) { returnContent(); } else { if (top.firstSupportPage == top.contentResourceID) { returnContent(); } else if (top.prevPage == quizID) { launchQuiz(); } else { launchReview(); } } } function launchReview(reviewID) { top.currentSupport = 0; chapterID = "knet-v214aCH47510"; nbparams = "CHAPID="+chapterID+"/RLOID="+"knet-v214aRLO47818"+"/RIOID="+reviewID; dash = chapterID.indexOf('-'); sub1 = chapterID.substring(0,dash); sub2 = chapterID.substring(dash+1); ContentRef = "../../../../../"+ nbparams +"/"+ sub1 +"/"+ sub2 +"/"; //alert(ContentRef); eval('top.frames.main_frame.location.href="'+ContentRef+'pageframesetrio.html"'); //added for navigating between Quiz and Review and back to regular Chapter pages top.currentSupport++; top.firstSupportPage = reviewID; top.prevPage = reviewID; top.contentResourceID = ''; } function launchQuiz(quizID) { top.currentSupport = 0; chapterID = "knet-v214aCH47510"; nbparams = "CHAPID="+chapterID+"/RLOID="+"knet-v214aRLO47821"+"/RIOID="+"knet-v214aRIO123683"; dash = chapterID.indexOf('-'); sub1 = chapterID.substring(0,dash); sub2 = chapterID.substring(dash+1); ContentRef = "../../../../../"+ nbparams +"/"+ sub1 +"/"+ sub2 +"/"; //alert(ContentRef); eval('top.frames.main_frame.location.href="'+ContentRef+'pageframesetrio.html"'); //added for navigating between Quiz and Review and back to regular Chapter pages top.currentSupport++; top.firstSupportPage = quizID; top.prevPage = quizID; top.contentResourceID = ''; } function returnContent() { rioID = top.currentPage; rloID = top.currentRLO; ContentID = top.currentChapter; top.firstSupportPage = ''; top.prevPage = ''; top.contentResourceID = ''; top.currentSupport = 0; nbparams = "CHAPID="+ContentID+"/RLOID="+rloID+"/RIOID="+rioID; dash = ContentID.indexOf('-'); sub1 = ContentID.substring(0,dash); sub2 = ContentID.substring(dash+1); ContentRef = "../../../../../"+ nbparams +"/"+ sub1 +"/"+ sub2 +"/" eval('top.frames.main_frame.location.href="'+ContentRef+'pageframesetrio.html"'); } </script> </head> <body leftmargin="20" topmargin="10" marginwidth="30" marginheight="10" bgcolor="#FFFFFF" onLoad="window.focus();" background="../../../../../images/ccna/common/lab/bglab.gif"> <table bgcolor="#000000" cellspacing="0" width="100%" border="0"> <tr> <td width="100%"> <table cellpadding="4" cellspacing="1" width="100%" border="0"> <tr> <td width="10%" bgcolor="#FFFFFF" valign="top"> <table cellpadding="0" cellspacing="0" width="100%" border="0"> <tr> <td width="33%"><script language="JavaScript"> //if (parent.parent.getTopName() == "chapterframeset") //{ var imgSource = '../../../../../images/ccna/common/lab/labback.gif'; document.write('<a href="javascript:PrevPage();">'); document.write('<img src="' + imgSource + '" border="0">'); document.write('</a>'); //} </script></td><td width="33%"></td> </tr> </table> <p> </p> </td><td bgcolor="#FFFFFF" valign="top"> <p align="left"> <span class="resourceTitle">10.7.7 Host/subnet schemes</span> </p><object height="100%" width="100%" codebase="http://active.macromedia.com/flash4/cabs/swflash.cab#version=4,0,0,0" classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000"><param name="movie" value="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123681/knet/v214aVID4934/10_7_7.swf"><param value="high" name="quality"> <embed height="100%" width="100%" pluginspage="http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash" type="application/x-shockwave-flash" quality="high" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123681/knet/v214aVID4934/10_7_7.swf"> </embed> </object>Instructor Note<p class="smtext">An unfortunate by-product of creating subnetworks is that the reserved network and broadcast numbers now exist for each and every subnetwork created. Thus entire blocks of IP addresses, which begin with these subnetwork ID and subnetwork broadcast numbers, are wasted. So the network administrator must strike a balance between the number of subnets required, the hosts per subnet that is acceptable, and the resulting waste of addresses.</p><p class="smtext">The "lab", a paper-based activity, requires approximately 45 minutes. This TI is related to CCNA Certification Exam Objectives #7, #29, #30, and #36.</p><p class="smtext">The optimal number of subnets and hosts must be determined before subnets are created.</p> <table cellpadding="0" cellspacing="0" border="0"> <tr> <td width="100%"><img height="10" width="1" border="0" src="../../../../../images/ccna/common/transdot.gif"></td> </tr> <tr> <td width="100%"> <table bgcolor="#000000" bordercolor="#000000" cellspacing="0" bordercolorlight="#000000" width="100%" border="0"> <tr> <td width="100%"> <table bgcolor="#FFFFFF" width="100%" cellspacing="0" cellpadding="3" border="0"> <tr> <td><span class="note"><b>NOTE:</b><p class="smtext">The number of subnets required in turn determines the number of hosts available. For example, if 3 bits are borrowed with a Class C network, only 5 bits remain for hosts.</p></span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><a href="#" onClick="javascript:launchReference('knet-v214aRIO125376', 'null', 'null');void(null);">Lab Activity</a><p class="smtext">This lab focuses on a Class C network with three subnets that is using a Custom Subnet Mask. It will help you develop a better understanding of IP subnet masks.</p><p class="smtext">You cannot use the first and last address within each subnet because one is the broadcast address of that subnet, and the other is part of the network address. When you create subnets, you lose quite a few potential addresses. For this reason, network administrators must pay close attention to the percentage of addresses that they lose by creating subnets.</p><p class="smtext"> <b>Example:</b><br>If 2 bits are borrowed with a Class C network, 4 subnets are created, each with 64 hosts. Only 2 of the subnets are usable and only 62 hosts are usable per subnet, leaving 124 usable hosts out of 254 that were possible before chosing to use subnets. This means 51 percent of the addresses are lost.</p><p class="smtext">This time borrow 3 bits, creating 8 subnets, of which only 6 are usable, with 30 usable hosts per subnet. This creates a total of 180 usable hosts, down from 254, but with a loss of only 29 percent of the addresses. Whenever subnets are created, consideration must be given to future network growth and the percentage of addresses that would be lost by creating subnets.</p><a href="#" onClick="window.open('../../../../../CHAPID=null/RLOID=null/RIOID=knet-v214aRIO123681/knet/v214aRIOv2141007/iaactivity.html','LabActivity','width=742,height=390,toolbar=0,scrollbars=0,left=26,top=38');void(null);">Interactive Lab Activity</a><p class="smtext">Subnetting Class C IP Addresses</p>Web Links<a target="_blank" href="http://oac3.hsc.uth.tmc.edu/staff/snewton/tcp-tutorial/"><span class="smtext">Introduction to the Internet Protocols</span></a></td> </tr> </table> </td> </tr> </table> </body></html>⌨️ 快捷键说明
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